1. Field of the Invention
This invention relates to a process for selectively depositing a conformal polymer coating on a substrate, such as a surface of a semiconductor device.
2. Description of the Prior Art
Various processes are known for depositing a conformal polymer coating, so as to mask selected areas of a substrate. Commonly used, for this purpose, are the poly-p-xylylene polymers, formed by condensation of p-xylylene monomer or its derivatives.
See U.S. Pat. No. 3,895,135, issued to Hofer on Jul. 15, 1975, in which a portion of a substrate, which is not to be coated with the polymer, is masked, with masking tape or the like, to form a constricted flow path for the vapor of the precursor. The coating forms over the mask, as well as the unmasked portions of the substrate; and the purpose of the constriction is to provide a thin coating at the masked/unmasked interface. The thin coating then serves as a tearline for removing the coated mask along the interface, leaving the coating only on the desired portions of the substrate.
See also U.S. Pat. No. 4,299,866, issued to Clark, et al. on Nov. 10, 1981, in which areas of a workpiece (such as the leads connected to a semiconductor chip), which are not to be coated with the polymer, are coated with a non-polymerizing hydrocarbon, e.g. Beeswax. The polymer coating forms over the workpiece, upon exposure to the vapor of the precursor, and openings are then formed in the coating. The workpiece is then immersed in a solvent, to swell the hydrocarbon, thereby causing removal of the coating overlying the hydrocarbon.
Both of these processes require, however, cumbersome sequences of mechanical steps, which deposit the coating everywhere and then remove it from the undesired portions of the substrate. This approach is particularly burdensome, in that poly-p-xylylene coatings adhere very tenaciously, making it difficult to remove them from the precise locations as desired.